The invention relates to a storage bunker, containing alumina and other additives, for a crust breaking facility which is used to break the solidified crust on an electrolytic cell, in particular on a cell for producing aluminum.
In the manufacture of aluminum from aluminum oxide the latter is dissolved in a fluoride melt made up for the greater part of cryolite. The aluminum which separates out at the cathode collects under the fluoride melt on the carbon floor of the cell; the surface of this liquid aluminum acts as the cathode. Dipping into the melt from above are anodes which, in the conventional reduction process, are made of amorphous carbon. As a result of the electrolytic decomposition of the aluminum oxide, oxygen is produced at the carbon anodes; this oxygen combines with the carbon in the anodes to form CO.sub.2 and CO. The electrolytic process takes place in a temperature range of approximately 940.degree.-970.degree. C.
The concentration of aluminum oxide decreases in the course of the process. At an Al.sub.2 O.sub.3 concentration of 1-2 wt.% the so-called anode effect occurs producing an increase in voltage from e.g. 4-4.5 V to 30 V and more. At this time at the latest the crust must be broken open and the concentration of aluminum oxide increased by adding more alumina to the cell.
Under normal operating conditions the cell is fed with aluminum oxide regularly, even when no anode effect occurs. Also, whenever the anode effect occurs the crust must be broken open and the alumina concentration increased by the addition of more aluminum oxide, which is called servicing the cell.
For many years now servicing the cell includes breaking open the crust of solidified melt between the anodes and the side ledge of the cell, and then adding fresh aluminum oxide. This process which is still widely practiced today is finding increasing criticism because of the pollution of the air in the pot room and the air outside. In recent years therefore it has become increasingly necessary and obligatory to hood over or encapsulate the reduction cells and to treat the exhaust gases. It is however not possible to capture completely all the exhaust gases by hooding the cells if the cells are serviced in the classical manner between the anodes and the side ledge of the cells.
More recently therefore aluminum producers have been going over to servicing at the longitudinal axis of the cell. After breaking open the crust, the alumina is fed to the cell either locally and continuously according to the point feeder principle or discontinuously along the whole of the central axis of the cell. In both cases a storage bunker for alumina is provided above the cell. The same applies for the transverse cell feeding proposed recently by the applicant (U.S. Pat. No. 4,172,018).
The known types of storage bunker or alumina silo on electrolytic cells are in the form of tapered funnels or containers with a tapered funnel in the lower part. The contents of the container or containers on the cell usually suffice for 1-2 days supply to the cell.
During the process of electrolysis the molten electrolyte becomes depleted not only in alumina but also in other additives such as cryolite and/or aluminum fluoride i.e. fluxing agents. In this case there are three ways which are known to supply the bath with the necessary additives:
The hooding over the cell is opened up and the additives fed manually or by means of a mobile servicing device when the crust is broken open. PA0 The additives are fed to the storage bunker via the supply line for the alumina. PA0 The additives are fed in a separate supply line to a hollow housing above the chisel of the crust breaker, from which they can be supplied to the bath (German Pat. No. 2,135,485). PA0 Each time the hooding over the cell is opened up fumes escape to the atmosphere in the pot room, which causes a deterioration in the working conditions in the pot room. PA0 If the additives are fed in a closed system into a storage bunker, then there can be a delay of up to a day or more before they reach the bath. This means it is not possible to ensure optimum operation of the cell. PA0 The arrangement using separate supply lines, pressurized containers, feeding devices and run outs requires much greater financial investment and technical effort. PA0 (a) The material supplied to flow through both end pieces into both containers. PA0 (b) The material supplied to flow through one end piece into the large or the small container. PA0 (c) Both end pieces to be closed off to prevent flow of material. PA0 (a) Position in silo not limited. PA0 (b) Better use of space. PA0 (c) Height can be varied; the mixing ratio can be adjusted by raising the tube.
However, all the known facilities for supplying additives to the cells exhibit disadvantages:
It is therefore the principal object of the present invention to provide a storage bunker for a crust breaking facility which is used to break the solidified crust on an electrolytic cell, and a process for supplying alumina, such that additives can be fed to the cell in a closed system without any significant additional costs and without the additives being delayed on route.